Moreover the middle ("Brace") node is a SmMathSymbolNode.

Change-Id: Ia0e4f798b69a9a205269bbd3f6c63d2059e8c766
Reviewed-on: https://gerrit.libreoffice.org/27769Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Takeshi Abe <tabe@fixedpoint.jp>